1) Field of the Invention
The present invention relates to a laser module in which an external cavity is formed with a Fabry-Perot semiconductor laser element and an optical feedback component, and more particularly, to a laser module that is ideal for a pump light source for an optical amplifier.
2) Description of the Related Art
With a widespread of the Internet and a rapid increase in connections among corporate local area networks (LANs), an increase in data traffic has become an annoying problem. Also, to prevent a decrease in communication performance, dense wavelength division multiplexing (DWDM) transmission systems have made remarkable progress and become a common standard.
In the DWDM transmission system, with a plurality of optical signals being carried on different wavelengths, a large-capacity transmission, which is hundred times as large as a conventional transmission, is achieved using a single fiber. Also, in the DWDM transmission system, to achieve a wideband transmission and a long-distance transmission, use of an optical amplifier is essential. Particularly, as the optical amplifier for the DWDM transmission system, an erbium-doped fiber amplifier (EDFA) is widely in use. The EDFA is an optical fiber amplifier in which a pump laser light having a wavelength of a 1480-nanometer band or a 980-nanometer band propagates through a special optical fiber doped with an erbium. With the pump light propagating through the special optical fiber, a light having a wavelength of a 1550-nanometer band, which is transmission-signal light, is amplified.
Thus, such an optical fiber amplifier includes a laser module for generating the pump laser light. To achieve optical communication with high quality and high reliability in the DWDM transmission system, stable optical amplification has to be performed on the transmission signal light, which means that high quality is required for the laser module mentioned above.
In the laser module for a pump light source, a condition for satisfying high quality is high degree of being monochromatic in a single longitudinal mode. As a light source of a laser module, a semiconductor laser element is normally used. However, a Fabry-Perot semiconductor laser element oscillates in a plurality of longitudinal modes (multimode), and therefore cannot sufficiently satisfy the required monochromatic characteristic. The requirement on the monochromatic characteristic is similarly applied to a laser module for a signal light source. Particularly, if the monochromatic characteristic is not satisfied in the laser module for a pump light source, a gain band with respect to the transmission-signal light becomes fluctuated, thereby making it difficult to amplify wavelength-multiplexed signal light as designed.
Therefore, to satisfy the required monochromatic characteristic, which means to achieve oscillation in a single longitudinal mode, a system has been developed for practical use in which a fiber Bragg grating (FBG) having a predetermined reflection bandwidth is provided on a transmission path of laser light emitted from a semiconductor laser element.
The operation of the structure using the FBG is briefly described. Of the laser light emitted from the semiconductor laser element, a portion of a wavelength band specified by a reflection bandwidth of the FBG is reflected to become returned light. This returned light is again fed back to the semiconductor laser element. That is, a reflection facet (rear facet) of the semiconductor laser element and the FBG form an external cavity. With this, the wavelength of the laser light emitted from the semiconductor laser element, that is, the wavelength of the exiting laser light emitted from the laser module, is stabilized at a specific value.
Also, in the EDFA described above, specification requirements of the laser module include a stable response-frequency characteristic in low frequencies (on the order of equal to or less than 100 kHz). That is, a laser module capable of outputting light with small low-frequency noise is desired. Therefore, in the semiconductor laser element normally adopted as a light source of the laser module, a measure for reducing low-frequency noise is taken.
Well-known causes of the occurrence of low-frequency noise in light output from the semiconductor laser element include fluctuations in oscillation wavelength and mode hopping. The fluctuations in oscillation wavelength are caused by changes in cavity length mainly due to changes in temperature. The intensity of the oscillation-wavelength light follows the gain spectrum of the semiconductor laser element, and therefore if the oscillation wavelength is fluctuated, the intensity of the light output is also changed. This change in the light intensity makes its presence known as the low-frequency noise described above. Here, a temperature-change ratio and a coefficient of linear expansion of a refractive index of the FBG are low compared with those of the semiconductor. Therefore, if the FBG is adopted, fluctuations in oscillation wavelength caused by the changes in cavity length described above can be prevented to some extent.
On the other hand, even if the FBG is adopted, the problem of mode hopping mentioned above still remains. A gain peak (center wavelength of a gain spectrum) of the semiconductor laser element is normally changed due to changes in temperature or changes in injection current. Therefore, in some cases, the gain peak is located approximately at the midpoint between adjacent two longitudinal modes defined by the cavity length. In such cases, these longitudinal modes are alternately switched as an oscillation wavelength. This phenomenon is called mode hopping mentioned above. Normally, the oscillation intensity differs among different longitudinal modes, and therefore noise occurs according to the difference in intensity when the oscillation wavelength is switched. This noise makes its presence shown as the low-frequency noise mentioned above.
To suppress the occurrence of the low-frequency noise due to mode hopping, contrarily to the monochromatic requirement mentioned above, it is known that oscillation in a plurality of longitudinal modes (multimode) is effective (see IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982). Specifically, the state of a carrier inside the semiconductor laser element is changed by optical feedback, temperature changes, or changes in injection current to the semiconductor laser element to destroy temporal coherency in a single longitudinal mode, thereby extremely shortening a coherence length in the longitudinal mode. As such, a phenomenon in which oscillation in a plurality of longitudinal modes (multimode) is caused by optical feedback, temperature changes, or changes in injection current to the semiconductor laser element is generally called a “coherence collapse”. Therefore, by causing such a coherence collapse, the occurrence of mode hopping can be reduced, thereby stabilizing the low frequencies of light output.
However, it has not been clear how much coherence collapse is required to stabilize the low frequencies of light output. A measure taken at present is merely causing a light output to be emitted for a predetermined period after the completion of the laser module for measuring temporal fluctuations in light output to see if the stability in low frequencies of the light output is within a predetermined range. In this measure, an index for determining the stability in low frequencies achieved by a coherence collapse cannot be obtained. Therefore, the stability check has to be performed on the completed laser module, which makes it difficult to select non-defective products at an early stage of the manufacturing process. Moreover, such a check requires a long time.
Also, to satisfy the required monochromatic characteristic by using the FBG in addition to oscillation in the longitudinal modes (multimode), it is preferable that the intensity in a longitudinal mode other than that of the FBG (main mode), particularly the intensity in oscillation wavelength (side mode) of the semiconductor laser element, is small. In other words, a ratio between the main mode and the side mode (side-mode suppression ratio: SMSR) is an important indicator. In the laser module, its design parameters are not independent from one another, but are closely related from one another. Therefore, if a parameter is changed to improve a certain characteristic, another characteristic may often be deteriorated. Therefore, it is difficult to find design parameters simultaneously satisfying a certain degree of a constant SMSR or higher with respect to various polarized states of laser light varying due to propagation through an optical fiber and the stability in low frequencies achieved by a coherence collapse.
Here, in a laser module using a GaAs-group semiconductor layer element, light-output fluctuations in laser light to be emitted are standardized to be equal to or less than 0.5%. Therefore, to satisfy this standard, unstable light outputs as described above pose a problem.